1. Field of the Invention
The present invention relates to a managing method of the start-up phase of at least one micro fuel cell to be connected to a load.
More specifically the invention relates to a method of the above type comprising the steps of:                measuring a micro fuel cell voltage value across said at least one micro fuel cell activated by means of fuel injection.        
The invention also relates to a managing circuit of the start-up phase of at least one micro fuel cell to be connected to a load.
The invention particularly, but not exclusively, relates to a method and circuit for managing the start-up phase of a stack of micro fuel cells and the following description is made with reference to this field of application by way of illustration only.
2. Description of the Related Art
As it is well known, a fuel cell substantially is an energy electrochemical converter which transforms the chemical energy of a fuel directly into DC electricity.
The operation mechanism of a fuel cell is based on a chemical reaction, wherein the elements at stake are a fuel and a comburent. The fuel can be hydrogen, methanol or other, while the comburent is oxygen. From this chemical reaction electricity, heat and water originate, as schematically shown in FIG. 1.
In particular, in this figure a fuel cell 1 is schematized which is supplied with hydrogen (H2) and oxygen (O2) and which produces electric energy (in particular electrons e−), water (H2O) and heat (cal).
The energetic profile of a generic chemical reaction can be represented by the following diagram:A+B→C+D  (1)
In general, in order for two substances (A+B) to react and give reaction products (C+D) it is necessary that these reactants impact against each other. Not all the impacts between reactants are reactive, but to the purposes of the obtainment of the products and from an energetic point of view only those occurring between molecules having a higher or equal energy with respect to an activation energy Ea are useful. The activation speed of a reaction is thus linked to the number of molecules having a higher or equal energy E with respect to the activation energy Ea (E≧Ea), as schematically shown in FIG. 2, where the energy E is indicated as a function of the reaction state, as well as a portion of energy Er released by the reaction itself. If the probability of having molecules with enough energy is low, then the reaction will proceed slowly, vice versa if the number of molecules is high.
It is possible to act on the activation speed by increasing the temperature of the reactants or by lowering the activation energy Ea by using catalysts, as it usually occurs in the use of fuel cell.
Notwithstanding the adoption of these measures, the attainment of the complete functionality of the fuel cells, is not, however, instantaneous; in particular, when a fuel cell is supplied with a fuel, the production of electricity is not immediate, but it effectively starts after a certain delay commonly indicated as “start-up” time.
In general, “start-up” indicates the time interval lapsing between the introduction of the fuel into a fuel cell and the stabilization of its static characteristic I-V.
The use is also widespread of micro fuel cells as portable supply sources for low power electronic devices. A central membrane coated on both sides by a catalyst layer is the core of a micro fuel cell; across the membrane the reaction occurs with the fuel on one side and with the oxygen present in the air on the other side and they can be realized with techniques known in the field of the microelectronics with extremely reduced dimensions.
In this case, among the factors influencing the start-up time there are also the moisturizing of the membrane, the diffusion time of the gases through a so called “gas diffusion layer”, and others more.
Micro fuel cells are normally organized in stack structures, where the cells are substantially overlapped on one another.
Considering the case of a stack of three micro fuel cells supplied with hydrogen, the progress of the open circuit voltage obtained across it during the start-up phase has a typical profile of the type shown in FIG. 3A, the stack of micro fuel cells reaching the steady state after a period equal to about 40 s.
Moreover, it is known that a stack of micro fuel cells shows a I-V (current-voltage) characteristic which is static—i.e., relative to all the possible working points of the stack—and stabilized—i.e., once the full operation condition has been reached—of the type shown in FIG. 3B. In particular, three operation areas are distinguished: an activation biasing region (A1), an ohmic region (A2), and a concentration biasing region (A3).
It is thus easily understood that, if a load is connected to a stack of micro fuel cells before the end of the start-up time, the time necessary for the micro fuel cells to reach their optimal operation condition increases and, at least initially, the stack cannot operate at the maximum of its potentiality.
In other words, the connection of a load to a stack of micro fuel cells before the start-up phase has been completed causes, as a matter-of-fact, the operation of the stack below its potentiality. The delicacy and the importance that this step holds are thus evident so as to operate with a perfectly stabilized stack of micro fuel cells.
To overcome this drawback, the solutions of the prior art being currently used in the field provide the use of power generators with functions of backup or emergency elements. In particular, systems for supplying micro fuel cells are known comprising at least one pre-charged buffer battery able to supply a load connected to the micro fuel cell, or the stack of micro fuel cells, during the start-up period, with enough power to allow a connection also in this initial operation phase of the micro fuel cells.
These supply systems comprising a buffer battery thus provide a connection of this latter to the load up to the completion of the stack start-up phase. The buffer battery supplies, in addition, the energy necessary to supply a control circuit supervising the start-up phase so as to connect the load to the stack only once it is stabilized.
Although advantageous under several points of view, these known solutions show several drawbacks. In particular, it is immediately evident that these supply systems are not able to manage a start-up phase of a stack of micro fuel cells in case this buffer battery is down as well.